The invention relates to a method for magnetic resonance spectroscopy (=MRS) or magnetic resonance imaging (=MRI) in which an NMR time-domain signal is created by an RF excitation pulse applied to an object in the presence of an applied magnetic field that may depend on spatial position and/or time, said time-domain signal being generated by an excited transverse nuclear magnetisation precessing about the applied magnetic field, whereby the RF excitation pulse is adapted to cover a whole range of NMR frequencies of interest present in the object, and time-domain signal acquisition takes place during, or during and after the application of the RF excitation pulse.
Such a method is known from U.S. Pat. No. 7,403,006 B2.
Magnetic resonance (MR) spectroscopy (MRS) or imaging (MRI) experiments are commonly performed with the pulsed Fourier transform (FT) technique. The spins in a sample are excited with a radio frequency (RF) pulse covering all frequencies required for the isochromats of interest. The term isochromats is used to denote groups of spins having the same resonance frequency. Such an RF pulse is typically amplitude-modulated and has a constant frequency and phase, and its duration is appropriate to provide the required frequency bandwidth. Sometimes, frequency- or phase-modulated pulses are also used. The excitation is followed by the acquisition of the MR signal emitted by the spins. Hence, excitation and acquisition are separated in time. Spectrum or image reconstruction usually includes Fourier transforming the time domain signal or similar procedures such as e.g. back-projection or more general algorithms for the inversion of the encoding procedure. All these types of reconstruction assume that all isochromats have the same phase after the excitation has been completed.
Less common than pulsed FT MR is the continuous wave (CW) technique developed at the beginnings of MR. In CW MR, the excitation is not completed before the data acquisition starts, but the signal bandwidth of interest is scanned with the radio frequency (RF) being swept over this frequency band. The sweep is slow enough to allow the spin system to reach an equilibrium state. The signal emitted by the spins is recorded simultaneously with the excitation. This is usually realized by using separate, decoupled transmitter and receiver coils. Here, the spectral data is obtained directly without the need for a Fourier transform.
In rapid scan correlation spectroscopy (Dadok 1974) the frequency sweep is performed faster than with CW MR and the spin system does not reach an equilibrium state. Assuming linear behaviour, the time domain signal can be considered a convolution of the impulse response of the spin system with the sweep pulse. Hence, the spectrum is obtained by de-convolution followed by Fourier transform or by division of the Fourier transformed signal by the Fourier transformed pulse.
The MRI equivalent of rapid scan correlation spectroscopy is the SWIFT technique (Idiyatullin 2006). A sweep pulse is applied while an imaging gradient is switched on, and the acquisition is performed simultaneously with the excitation. This can be accomplished by using either decoupled transmit and receive circuits or interleaved pulsing and signal reception. Reconstruction of an image profile is performed in analogy to the spectroscopy method. A three-dimensional (3D) image is obtained by application of a series of gradient directions, each providing a different radial projection of the object, followed by a suitable 3D reconstruction algorithm, such as e.g. interpolation onto a Cartesian grid followed by 3D Fourier transform.
The SWIFT method may be regarded as an application of frequency swept excitation pulses that allows the reduction of the peak RF power while keeping the required excitation bandwidth. The simultaneous signal acquisition permits the detection of spins with short transverse relaxation times despite the extended pulse duration.
Sweep pulses are also used in a technique published by Pipe (Pipe 1995), however they are applied before data acquisition. A linearly swept pulse is followed by an RF- or gradient refocusing and then by the data acquisition. This technique uses a quadratic dependence of the transverse magnetisation phase on the resonance frequency offset to time-encode the position of the signal sources and does not require the Fourier transform. Its resolution is lower than that of the Fourier transform-based methods with identical detection times.
The sweep pulses with quadratic phase profiles may also be used for slab selection in classic 3D Fourier-encoded experiments to distribute the signal power to several encoding steps (Park 2006). This has the advantage of reducing the dynamic range of the signal making it less prone to quantisation noise introduced by the analogue-to-digital conversion. The reconstruction of the image uses the discrete 3D Fourier transform.
In certain situations, the reconstruction of the image or spectrum by means of the Fourier transform is not optimal or not possible. These situations include e.g. imaging experiments with undersampled non-Cartesian trajectories using array detection (Pruessmann 2001), or acquisitions with missing samples after the excitation pulse (Hafner 1994, Kuethe 1999).
It is the object of the invention to provide an improved method for reconstructing spectral or image data from time-domain signal obtained with simultaneous excitation and acquisition which can be used more versatilely than conventional Fourier transform.